1. Field of the Invention
The present invention relates generally to aircraft automatic flight control systems, and more particularly to an automatic throttle position control system for achieving and maintaining a commanded cruising airspeed during the capturing of a desired altitude from either ascending or descending flight.
2. Description of the Prior Art
Most modern commercial transport aircraft, many general aviation aircraft, and some millitary aircraft are equipped with automatic flight control systems which generally assist the human pilot in efficiently maneuvering the aircraft from a state of ascending or descending flight to achieve a commanded cruising altitude. In addition, many aircraft are equipped with automatic throttle control systems which manipulate the aircraft engine throttles to achieve a desired thrust level or to capture and maintain a manually selected or computed airspeed. Aircraft having both automatic flight control systems and autothrottle systems are referred to as fully coupled, whether the systems work independently or in a complementary mode, as in performance management systems for controlling the vertical path of the aircraft or flight management systems for controlling both the vertical and horizontal path of the aircraft.
In view of the substantial increases in fuel costs, aircraft operators are very desirous of increasing fuel efficiencies throughout the entire vertical flight profile of their flight plans by assuring the most cost-effective operations possible. A particular objective of such operations is to obtain smooth, stable and accurate airspeed control during cruise and when the automatic flight control system commands the aircraft to accelerate, decelerate or change its flight path in the vertical plane. Further, during the capture of a cruise altitude, it is desirable, if not mandatory, to maintain the aircraft's speed within predefined limits. Typical of such cases is that where the pilot is mandated by Air Traffic Control to decelerate to a specified speed at a specified altitude in order to maintain air traffic separation. For example, in the United States the maximum speed of aircraft is restricted by the Federal Aviation Administration of 250 nautical miles per hour in flight below 10,000 feet of altitude. Thus, should the human pilot elect to capture a cruising altitude less than 10,000 feet of altitude while climbing at 250 nautical miles per hour, it is necessary that the aircraft's speed not exceed the speed restriction during the capture manuever and subsequent cruise at the selected altitude. Once the aircraft has reached the desired altitude, the engine thrust must be increased or reduced to that value which will maintain the desired airspeed.
In the prior art, control of speed during the altitude capture maneuver was generally performed by adjusting the throttles in a manner proportional to airspeed error in an attempt to minimize speed deviations. Such schemes are well-known to those skilled in the art and are generally of the classical proportional plus integral or proportional plus derivative (or a combination of both) servo control systems with airspeed as the controlled parameter. The proportional and integral system tends to reduce steady-state errors, while the proportional and derivative system has no effect on steady-state errors and reduces transient errors.
Since the time constant of the airspeed control response associated with the autothrottle commands is of the order of 5-10 seconds, an accurate, responsive control function requires an input that anticipates the airspeed change resulting from altitude change maneuvers. The shortcoming of many prior art systems is that they are purely reactive in nature; i.e., a significant airspeed error is required before corrective action is taken by the autothrottle system. Thus, highly responsive systems generally result in unacceptable throttle activity, while highly filtered systems are sluggish and allow large speed deviations to occur before corrective action is taken and may lead to instability.
Other prior art systems, particularly performance management systems and flight management systems, generally compute the thrust required to maintain the cruise speed, usually expressed as an operational characteristic of the engine, such as engine pressure ratio (EPR) or engine fan speed (N.sub.1), and drive the throttles at a constant rate to achieve the desired cruise thrust setting during the altitude capture maneuver. The shortcoming of this scheme is that the constant throttle rate does not assure adequate speed control for all capture manuevers because of the associated change in pitch attitude. For example, large speed losses can occur if the capture manuever is begun at large rates of climb or descent and, similarly, excessive overspeed conditions can occur if the capture maneuver is initiated at relatively small rates or climb or descent. Further, if a high rate of throttle change is commanded, the resulting acceleration may prove uncomfortable to passengers.
The present invention overcomes the shortcomings of the prior art systems by manipulating the throttles at a variable rate responsive to the actual capture maneuver itself. The throttles are restricted to rates dependent on the difference between the instantaneous actual thrust of the aircraft (in EPR) and the final cruising thrust and upon the actual altitude rate to automatically advance or retard the engine throttles in such a manner as to assure that the rate of throttle movement maintains the speed of the aircraft throughout the automatic altitude capture manuever.